This invention relates to implantable diagnostic and chronic therapeutic leads, and more particularly to cardiac pacing leads capable of delivering drugs near the point of implant.
Cardiac pacing leads are recognized as well suited for carrying pulse stimulation signals to the heart from a pacemaker, and for monitoring heart electrical activity from outside the body. Typically, such leads are sufficiently flexible and small in diameter for intravenous introduction to a cardiac cavity, whereupon an electrode at the distal end of the lead is implanted into the endocardium to secure the lead. A persistent problem accompanying the use of pacemaker leads is the increase in stimulation threshold, both acute and chronic, caused by the interaction between the electrode and body tissue at the point of implant. Approaches to reducing the threshold, or conversely to increase the lead sensitivity, include introduction of thin, limp leads, small porous electrodes, electrodes with activated surfaces, and electrodes of "biocompatible" materials such as carbon.
Relatively recent among these approaches is a local introduction of a drug for reducing the stimulation threshold. For example, U.S. Pat. No. 4,506,680 (Stokes) shows a body implantable lead with a silicone rubber plug housed in a cavity within the lead and impregnated with dexamethasone sodium phosphate. An elution bore is filled with sintered metal microparticles in order to control the rate of drug elution through the use of this densely packed medium. A sealing washer, backed by a metal disc, separates the distal and proximal cavities in the crimp tube, with the drug plug in the distal cavity. U.S. Pat. No. 4,557,642 (Stokes) shows a crimp tube having a medial wall to form in the crimp tube a distal cavity to house a drug matrix, mainly a powdered form of molecular sieve material. Side apertures are provided in the crimp tube for loading the drug. These devices, while suitable for certain applications, fail to address the need for rapid drug delivery to counter acute threshold increase, for selectively combining drugs for either sequential or simultaneous local delivery, or for convenient loading of a drug delivery matrix into a lead.
Therefore, it is an object of the present invention to provide an implantable cardiac pacing lead adapted for rapid drug delivery immediately upon implant to counter an acute stimulation threshold increase.
Another object is to provide a cardiac pacing lead having a porous electrode capable of holding a drug in liquid form or solid form for acute local delivery upon implant, in combination with a drug impregnated matrix for chronic drug delivery.
Another object of the invention is to provide a pacemaking led employing a plurality of drug matrices for selectively varying the type of drug, concentration of drug and drug delivery rate.
Yet another object is to provide a body implantable lead enabling convenient, proximal end loading of one or more drug impregnated matrices into structure coupling the lead electrode with the distal end of the lead conductor.